Fundamentally, nothing really. It's all the same: money moves from one person to another—quickly, instantly, and no fees (unless it's a business transaction)—as long as it doesn't leave the network. But therein lies the rub: money has to leave this network occasionally because you cannot send money peer-to-peer to from PayPal to a merchant not using PayPal (like Amazon), and that is the troublesome part where money has to be ACH'd to bank accounts.

And as such, the p2p ecosystems differentiate themselves by giving people ways to pay merchants without explicitly needing to move that money out of the network. So, Cash App issued a debit card, and then PayPal, and then Venmo (yep, even though it's a subsidiary, it's a separate card). And then some of them started using real bank accounts (with routing/account numbers) to back the stored balance. And the result today is that all these p2p apps look awfully a lot like banks, but less encumbered by regulations because they don't do everything that banks do.

Back to your question, why bother changing to Venmo (or Cash App) when PayPal is basically the same at this point? Obviously, the primary reason would be if everyone you know is using not-PayPal. A secondary reason is the same reason that people have different credit cards from different banks: different rewards schemes (yep, there are incentives for using those p2p debit cards).

Zelle is probably the odd one out here, because it's just an extra thing built on top of existing participating banks that already have all these ways of moving money out of their networks. It's effectively a wire transfer replacement without the fee (or a physical check replacement without the check-writing).

(Disclosure: am employed by a company that has a p2p product.)